How do I maintain my boner?

I'm 26 and probably going to lose my v-card tomorrow. I have a little trouble getting it up sometimes, and keeping it going. It usually takes me 15-20 minutes to finish masturbating due to an SSRI I'm on. I stopped taking Benadryl because I was pretty sure that was contributing to my problem.

Usually what happens when masturbating
>Not hard for 30 seconds
>Semi hard for a couple minutes
>Fully hard
>Semi hard for 8-10 minutes
>Fully hard when about to cum, then cum

Is this normal considering I last a really, really long time?

Repost from /adv/ because that board is dead.

Also r8 my cock